Field of the Invention:
The present invention relates to a Cu core ball, a solder paste and a solder joint having an improved surface roughness and low alpha dose.
Description of Related Art:
Recently, along development of compact information equipment, electronic components to be mounted on them have been downsized rapidly. A ball grid array (hereinafter referred to as “BGA”) having electrodes at its rear surface is applied to such electronic components in order to satisfy a requirement of a narrowed connection terminal and a reduced mounting area because of the downsizing requirement.
In the electronic components wherein the BGA is applied to its semiconductor package, a semiconductor chip having electrodes is sealed with resin and solder bumps are formed on the electrodes of the semiconductor chip. The solder bump is that a solder ball is joined to the electrode of the semiconductor chip. This solder bump is connected to a conductive land of a printed circuit board so as to be mounted on the printed circuit board.
Recently, a three-dimensional high-density package is studied by stacking up the semiconductor packages in a height direction in order to meet the further high-density mounting.
In a case that the BGA is applied to the semiconductor package for the three-dimensional high-density mounting, the solder ball may be crushed by a semiconductor package's weight. If such an accident happens, the solder may be forced out so that a Short circuit may happen between the electrodes.
In order to resolve the above-described problems, it is considered to use a ball whose hardness is higher than that of the solder ball. As the ball having a higher hardness, a solder bump using a Cu ball or Cu core ball is reviewed. The Cu core ball is that a coating (solder plating film) is formed on a surface of the Cu ball.
Since the Cu ball and the Cu core ball do not melt at a melting temperature of the solder, the solder bump does not crush at a mounting process even if the solder bump receives the weight of the semiconductor package. Therefore, the semiconductor package can be supported very well. Such technology associated to the Cu ball and the like is disclosed, for example, in International Patent Publication No. WO/1995/24113 (hereinafter referred to as “WO/1995/24113 publication”).
By the way, the downsizing of the electronic components allows the high-density mounting, but the high-density mounting causes soft error problems. The soft error is a possibility of rewriting a storage content of a memory cell in a semiconductor integrated circuit (IC circuit) with alpha ray entering into the memory cell.
It is conceivable that beta decay occurs on a radioactive element such as U, Th, Po, etc. or a radioactive isotope included in Pb, Bi, etc. in the solder alloy and then alpha decay occurs so that the alpha ray is emitted. Recently, a low alpha ray solder material is developed wherein the contained amount of the radioactive element is reduced. An associated technology is disclosed, for example, in Japanese Patent No. 4,472,752 (hereinafter referred to as “JP/4,472,752 patent”).
In the Cu core ball, the solder plating film is formed on the surface of the Cu ball. This solder plating film is a plated layer having a uniform film thickness. However, a growth of a crystal may be non-uniform based on a condition of the plating process and like. As a result, the surface thereof may be uneven. The Cu core ball having the uneven surface does not roll smoothly. Therefore, when the ball is jointed as the solder bump, an accuracy of the joint position becomes low.
In addition, organic constituent maybe caught up in the uneven surface during a period of forming the bump and the organic constituent may be molten and gasified during the reflow. This may remain as void in the solder plating film. The remained void causes trouble such as a reduced reliability of the joint or a position gap of the solder bump at a time when the gas composition is discharged from the solder plating film.
Japanese Patent Application Publication No. 2010-216004 (hereinafter referred to as “JP/2010-216004 publication”) or the like discloses a technology of improving the unevenness of the solder plating film.
By the way, the WO/1995/24113 publication discloses the Cu ball and the Cu core ball having the higher sphericity. However, this publication does not disclose the alpha dose of the Cu core ball.
Moreover, the WO/1995/24113 publication describes only Pb—Sn alloy as the solder alloy of making up the solder in the background explanation. The alpha ray is emitted from an isotope 210Pb of Pb included as an impurity in Sn along the decay process 210Pb->210Bi->210Po->206Pb. Alpha ray are generated by the disintegration of polonium Isotope 210Po to Isotope 206Pb.
It is conceivable that the Pb—Sn solder alloy contains its radioisotope 210Pb because the Pb—Sn solder alloy contains Pb in large quantities wherein this solder alloy is only one disclosed in the WO/1995/24113 publication. Therefore, even if this solder alloy is applied as the solder film of the Cu core ball, it is impossible to reduce the alpha dose. The WO/1995/24113 publication does not disclose a Sn plating on the Cu ball and an electrolytic plating under a flowage condition of the Cu ball and electrolysis solution.
In electrolytic refining described in the WO/1995/24113 publication, since an electrolytic deposition surface is limited to one direction, it is impossible to form a plating film having an even film thickness on a micro work piece such as the Cu ball.
The JP/4,472,752 patent discloses a technology of Sn ingot having a low alpha dose wherein it does not only electrolytically refine but also reduces the alpha dose by suspending adsorbent in electrolyte solution to adsorb Pb and Bi.
According to the JP/4,472,752 patent, since a standard electrode potential of Pb or Bi is close to that of Sn, it is difficult to reduce the alpha dose only by depositing electrolytically Sn on a plane electrode with a general electrolytic refining. Assuming that the electrolytic refining described in the JP/4,472,752 patent is applied to a process of forming the plating film on the Cu ball and the adsorbent is suspended in the plating solution for the barrel plating, the adsorbent is stirred simultaneously with the stir of the plating solution and the work piece. In this case, there is a possibility that the Pb ion and Bi ion absorbed on the adsorbent become carriers and are incorporated into the solder film with the adsorbent.
The solder plating film, which takes in the adsorbent, emits the high alpha ray. Since the adsorbent has a very small grain diameter of the order of sub-micron, it is difficultly conceivable to separate and collect the adsorbent after the suspension while flowing the plating solution. Therefore, it is difficult to prevent the adsorbent, to which Pb and Bi are absorbed, from being incorporated into the film.
In addition, the WO/1995/24113 publication discloses a Pb—Sn based solder alloy. However, since a plating method, a deposit method, a brazing method and the like are described as an equivalent method, this publication does not disclose a reduction of the alpha dose for a Sn based solder.
The subject matter of the WO/1995/24113 publication is to manufacture the Cu core ball having the high sphericity. The JP/4,472,752 patent discloses that the alpha dose is reduced by removing Pb from Sn-base wherever possible in the electrolytic refining process.
Therefore, a person skilled in the art, who knows the WO/1995/24113 publication, cannot suppose a problem where it is required to reduce the alpha dose from the Cu core ball disclosed in this publication. In addition, the composition of the Pb—Sn solder is different from that of the Sn based solder. Thus, it is conceivable that huge numbers of trial-and-error processes are required to suppose the problem of reducing the alpha dose of the Sn based solder and to select the Sn based solder from an endless number of solder alloys instead of the Pb—Sn solder alloy for the solder film.
It may be very difficult for those skilled in the art to make the plating solution by using the Sn ingot having the lower alpha dose described in the WO/1995/24113 publication and to form the Cu core ball with the plating method described in this publication.
If a joint is formed by using the Cu core ball manufactured with the prior arts described in the WO/1995/24113 publication or the JP/4,472,752 patent, there is a high possibility that radioactive elements in the solder film of the Cu core ball diffuse into the electrodes of the joint and then the alpha ray is emitted. Therefore, the soft error as a new problem of the high-density mounting cannot be solved.
The JP/2010-216004 publication discloses the improvement of the surface roughness of the solder plating film by using a smoothing process wherein media such as abrading agent contact with the surface of the solder plating film. The disclosed media are a physics method of mechanically removing as described hereinbefore and a chemical method such as acid wash.
However, it takes very long hours for the smoothing process with such media to accomplish the surface roughness (Ra) enough for the mounting process. For example, 5 through 6 hours are required for accomplishing the arithmetic average roughness Ra of the above surface roughness equal to or less than 0.3 μm. However, a practical realization of such surface roughness is difficult.